The present invention relates to a method and device for controlling a diesel engine.
Diesel engines mounted in an automobile may inject fuel two or more times into cylinders during one engine cycle in order to reduce NOx and soot in exhaust gas, reduce noise and vibration, improve a fuel consumption and torque. For example, JP2009-293383A discloses a diesel engine with a turbocharger in which fuel injections are carried out at five timings. The five injections are a main injection for torque generation, and a pilot injection performed prior to the main injection in order to preheat cylinders, a pre-injection for suppressing an ignition delay of fuel due to the main injection between the pilot injection and the main injection, an after-injection for increasing an exhaust gas temperature after the main injection, and a post-injection for directly introducing fuel into an exhaust system after the after-injection and increasing a catalyst temperature.
Meanwhile, in the fuel injection mode disclosed in JP2009-293383A, the main injection injected near a top dead center of a compression stroke relates to a diffusion combustion in which an injection of fuel, and an ignition and combustion of the fuel are performed in parallel. Unlike such diffusion combustion, a premixed charge compression ignition (PCI) combustion (hereinafter, simply referred to as “premix combustion”) mode is known, in which, after the injected fuel and air are fully mixed, the mixed gas is ignited and combusts near a top dead center of the compression stroke. Such a premix combustion mode suppresses generation of soot and NOx, and is utilized as a combustion mode excelling in an NVH (Noise Vibration Harshness) performance with a slow-down effect of the combustion. For example, it is performed in an operating range of comparatively low load in which a long delay time (ignition delay) from the injection of the fuel until the fuel is ignited within the cylinder can be secured.
However, when the engine load increases and a fuel injection amount is increased, the ignition delay becomes gradually shorter. Especially in an engine with a turbocharger, a supercharging volume increases with the increase in the load, and a temperature and a pressure at the end of the compression stroke rise, thereby the ignition delay further becomes shorter. As a result, the sufficiently long ignition delay cannot be secured and the premix combustion will not be established. That is, the operating range where the premix combustion mode which is advantageous for emission performance and NVH performance can be carried out is limited to a part of the range on a low-load side and, thus, it is disadvantageous because the operating range is comparatively narrow.